Runoff coefficient can be used as a measure for assessing the flood generation potential of the watersheds. This factor is depending on the watershed characteristics and may vary temporally during a year. Knowledge on the watersheds with higher potential to runoff production and determining pick time of runoffs is essential for designing watershed management practices in an area. In this study, physical characteristics with rainfall and runoff data for a thirty-year period (1986-2010) were studied in eleven forth order watersheds in Ardabil, Iran. The watersheds surface areas vary from 257 to 2184 km2 and their slopes are between 16.25% and 33.3%. Significant differences were found among their monthly rainfall (p<0.001), runoff (p<0.01), and runoff coefficient (p<0.05). There were significant relationships between runoff and rainfall in some watersheds which could be related to the poor vegetation cover. In most of the watersheds, the highest runoff coefficient (70 percentage) was appeared in April while the lowest value (zero) occurred in September. This result was associated with the shorter intervals between the rainfalls and poor vegetation cover in early spring. Suitable vegetation cover decreased the runoff coefficient in September. Runoff coefficient was significantly affected by the watersheds characteristics: the land surface area (r= -0.80), waterways length (r= -0.83) and form factor (r= -0.83). There were no significant correlations between runoff coefficient and land slope, Gravelious coefficient, and stream density. Results showed that in the semi-arid watersheds areas, the largest flood hazard belonged to the watersheds with lower area and non-elongated shape (Gravelious coefficient= 1.2) especially in early spring.